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Thread: Spectrum Plot

  1. #1
    impulse Guest

    Default Spectrum Plot

    Hi All,

    A Network Admin friend of mine mentioned that he uses Chanalyzer for surveying/troubleshooting his WiFi networks. I was intrigued and have a few questions about exactly what is being measured by your software;

    - The amplitude versus frequency plot appears to be a power spectral density (PSD) plot, similar to what a spectrum analyzer would produce. However I am curious as to what the radio bandwidth (commonly referred to as RBW) is for this measurement?

    - Is the Signal Strength (dBm) a channel power measurement determined from the PSD plot, or is it simply inferred from an RSSI value reported by the NIC?

    Thanks.
    Last edited by impulse; 03-29-2010 at 08:03 AM.

  2. #2

    Default

    Ok, I'm not 100% sure that I'm accurate, so don't hold me to this:

    The bandwidth is adjustable(WiSpy 2.4x, DBx, and 900x.), but it alters the sweep time.(lower bandwidth = longer sweep time)

    The signal strength is calculated in Chanalyzer using strength data from the WiSpy. It will only use a WiFi adapter for SSID overlays.

    I hope it helps.

  3. Default

    Quote Originally Posted by impulse View Post
    Hi All,

    A Network Admin friend of mine mentioned that he uses Chanalyzer for surveying/troubleshooting his WiFi networks. I was intrigued and have a few questions about exactly what is being measured by your software;

    - The amplitude versus frequency plot appears to be a power spectral density (PSD) plot, similar to what a spectrum analyzer would produce. However I am curious as to what the radio bandwidth (commonly referred to as RBW) is for this measurement?

    - Is the Signal Strength (dBm) a channel power measurement determined from the PSD plot, or is it simply inferred from an RSSI value reported by the NIC?

    Thanks.
    First and foremost, feel free to download Chanalyzer 3 and open up some of the recordings yourself. I think using it will help you answer many of your own questions.

    Chanalyzer 3 Download Link

    You'll see that you have a lot of Spectrum Analyzer tools built in like inspector and hardware config.

    The default bands can be found on this page:
    Compare Wi-Spy DBx, Wi-Spy 2.4x, Wi-Spy 2.4i and Wi-Spy 900x

    The Wi-Spy 2.4x and DBx can scan from 2300MHz to 2600MHz and the Wi-Spy DBx can scan from 4900MHz to 6000MHz. All of the MHz ranges can be adjusted by step size and resolution bandwidth. Please note that while you can scan outside of the public ISM bands, we don't test those ranges as rigorously.

    I'm not sure where you are seeing "signal strength" but if this is in our software called inSSIDer this would be from the NIC. Because the Wi-Spy is a Spectrum Analyzer, we use Amplitude (dBm).
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  4. #4
    impulse Guest

    Default

    That's odd - it seems my follow-up post vanished. Nevertheless, my questions/thoughts in brief;

    - Radio Bandwidth / Resolution Bandwidth (RBW) are effectively the same thing. It's important that these can be adjusted and it seems Chanalyzer allows you to do so - good stuff!

    - I picked up the term "Signal Strength" from inSSIDer. I am curious as to how the Signal Strength value reported by the NIC compares to the Power Spectral Density (PSD) measurements taken by Chanalyzer (and by spectrum analzyers in general). Results of my investigations thus far indicate that Signal Strength is likely to be a narrowband PSD measurement at the channel centre frequency (i.e. P(dBm)/RBW(Hz)), rather than a wideband channel power measurement (i.e. P(dBm)/22MHz). You guys have any thoughts/further insight on this?

    Edit: I am also interested as to how inSSIDer converts the RSSI (8-bit integer) reported by the NIC to RSS (dBm), particularly in those cases where there is no vendor look-up table for the conversion.

    Thanks.
    Last edited by impulse; 04-05-2010 at 11:13 PM.

  5. #5

    Default

    We need to change the label to "dB RSSI" or something like that, because it's not actually in dBm. Since we don't have/want to use a lookup table to convert the RSSI for every WiFi adapter out there, that would be the best road to take

  6. Default

    Quote Originally Posted by impulse View Post
    That's odd - it seems my follow-up post vanished. Nevertheless, my questions/thoughts in brief;

    - Radio Bandwidth / Resolution Bandwidth (RBW) are effectively the same thing. It's important that these can be adjusted and it seems Chanalyzer allows you to do so - good stuff!

    - I picked up the term "Signal Strength" from inSSIDer. I am curious as to how the Signal Strength value reported by the NIC compares to the Power Spectral Density (PSD) measurements taken by Chanalyzer (and by spectrum analzyers in general). Results of my investigations thus far indicate that Signal Strength is likely to be a narrowband PSD measurement at the channel centre frequency (i.e. P(dBm)/RBW(Hz)), rather than a wideband channel power measurement (i.e. P(dBm)/22MHz). You guys have any thoughts/further insight on this?

    Edit: I am also interested as to how inSSIDer converts the RSSI (8-bit integer) reported by the NIC to RSS (dBm), particularly in those cases where there is no vendor look-up table for the conversion.

    Thanks.
    This is odd, because I remember responding to that post as well. You asked if Chanalyzer measures power per Wi-Fi channel. Other than the inspector measurements the Wi-Fi report does do a wideband measurement.

    See image attached.

    Also I know why the posts might have disappeared. We had to revert the website to a sunday backup mid-Monday. Any posts between those times were probably lost.
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  7. #7
    impulse Guest

    Default

    Quote Originally Posted by Trent View Post
    This is odd, because I remember responding to that post as well. You asked if Chanalyzer measures power per Wi-Fi channel. Other than the inspector measurements the Wi-Fi report does do a wideband measurement.

    See image attached.

    Also I know why the posts might have disappeared. We had to revert the website to a sunday backup mid-Monday. Any posts between those times were probably lost.
    It's good to know that Chanalyzer also measures channel power - it's useful for link budget calculations.

    I'm not sure whether you caught my last question so I'll reiterate; how does the RSSI value reported by the NIC compare to the Chanalyzer channel power measurement and the Chanalyzer average/peak PSD measurement. The reason I ask is because I'm wondering whether the NIC RSSI is based on a peak/average PSD measurement (narrowband) or a channel power measurement (wideband).

    The IEEE 802.11 specification states that the RSSI is a measure by the PHY of the power observed at the antenna (to receive the current PPDU). It doesn't actually specify what power is measured - i.e. the channel power or the peak/average PSD. With this in mind I suppose the RSSI could be either, depending on how the NIC vendor measures the "power observed at the antenna".

  8. Default

    Well, I can't say how the NIC will compare to the Wi-Spy power readings. Because each vendor does it differently. But they do base their readings on the channel power measurement (average wideband).

    The Wi-Spy will give a more accurate power reading than any NIC because it is calibrated to function as a spectrum analyzer. The readings of the Wi-Spy DBx will be within +/-1.5dB.

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